The need for corona detection is widespread and extends not only to power utility companies, but also to any entity interested in maintaining power circuitry and electronic devices. Corona discharge is a luminous partial discharge from conductors and insulators caused when an electric field surrounding the conductors and insulators exceeds a critical value and causes the surrounding air to begin to behave more as a conductor rather than an insulator. A high local electric field ionizes the air, causes excitation of nitrogen molecules in the air, and leads to discharge of ultraviolet radiation.
Corona discharge can be problematic for several reasons. For example, corona discharge generates ozone and nitrogen oxides, which can form corrosive compounds such as nitric acid, especially in the presence of high humidity. These corrosive compounds can significantly reduce the service life and performance of electrical components. Additionally, corona discharge can cause damage to high voltage insulators and can create radio interference and audio noise.
Electric fields large enough to cause ionization of the air are commonly caused where there is a problem or defect associated with power supply circuitry or other electrical components. These problems and defects may correspond to points of potential equipment failure. Thus, devices configured to detect the presence of corona discharge can be used to locate and correct such problems and defects before failure occurs.
Corona discharge emits radiation within a spectral range of about 280 nm to about 400 nm. This spectral region falls mostly in the UV range and, therefore, is invisible to the human eye. Light emitted by corona discharge in air is heavily concentrated in a relatively small number of very narrow bands, typically a few nanometers (nm) or less, with weak emission intensity between the peaks of these bands. More than half of the total intensity emitted by a corona discharge is emitted at wavelengths shorter than 380 nm, and the strongest emission occurs at a narrow band centered about 337.1 nm. Additionally, reasonably strong emission occurs at shorter wavelengths, down to about 295 nm.
Various prior art devices have been developed and proposed for detecting and/or imaging areas of corona discharge. Generally, these devices included one or more UV filters for isolating UV radiation spectral bands that include corona discharge. In this manner, an image of the corona discharge could be generated based on the filtered UV radiation.
These prior art devices, however, were problematic. For example, the filters of these devices passed little or no radiation outside of the UV spectral bands specific to corona discharge. Therefore, these devices either completely lacked the ability to generate images based on radiation in the visible spectrum or produced visible images of poor quality. As a result, while these devices may have been able to image areas of corona discharge, they were less useful in providing a reference image (e.g., a visible radiation-based image) to enable a user to determine where on an object an imaged corona discharge originated.
Further, other prior art devices employed complex schemes of multiple filters to generate images of corona discharge. Each of the filters included a narrow passband intended to capture radiation emitted only at a particular spectral line of the corona discharge emission spectrum. Such multiple filter schemes can add to the expense and complexity of the corona detector design and degrade performance by introducing a plurality of attenuation sources in the optical path of the detector.
Still other devices included a single filter restricted to a narrow passband centered about a particular corona discharge spectral line. These devices had little flexibility. While they may have been able to generate an image of corona discharge, they were often restricted to certain operating conditions. Because of interference caused, for example, by the presence of solar radiation, a corona detector sensitive to only a particular spectral band may be useful only for night time or indoor use. Installation of a different filter targeting a different spectral line may be required for using the same corona detector outdoors or during daylight hours. Therefore, not only did these devices lack flexibility, but they also were unable to produce suitable reference images based on visible radiation.
Other prior art devices attempted to solve the problem of providing a user with a suitable reference image based on visible radiation by generating a visible image and merging this image with a UV image of the corona discharge. This approach typically required mirrors, multiple filters, and/or multiple optical paths. Further, this approach required multiple imaging devices and complicated optics or processing circuitry to overlay or merge the visible and UV images.
The disclosed corona detection device is directed toward overcoming one or more of the problems described above.